Active matrix organic light emitting diode (AMOLED) display is one of the hotspots in display research field. Compared with liquid crystal displays, organic light emitting diodes (OLED) have the advantages of low energy consumption, low production cost, self-illumination, wide viewing angle, fast response and etc.
In AMOLED display products, the brightness of a pixel is determined by the amount of current flowing through the organic light emitting diode, and the amount of the current is controlled by a driving thin film transistor in the pixel driving circuit. In the case the displays have the same brightness, as pixels per inch (PPI) of the display increases, the current required by each pixel becomes smaller and smaller, which requires a width-to-length ratio (W/L) of the channel of the driving thin film transistor in the pixel driving circuit to be reduced. Due to the width of the channel of the driving thin film transistor is limited by manufacturing process, the width-to-length ratio (W/L) of the channel of the driving thin film transistor is usually reduced by increasing the length of the channel, thereby reducing the current of the organic light emitting diode. However, high PPI, high resolution and high refresh rate are trends in AMOLED display products, high resolution and high refresh rate will shorten the charging time of each row of the pixels, which will affect the charging rate of the gate of the pixel driving thin film transistor. In order to increase the charging rate, it is necessary to increase the width-to-length ratio of the channel of the driving thin film transistor.